The present invention relates to internal combustion engines, and, more particularly, to exhaust gas recirculation systems used with internal combustion engines.
An exhaust gas recirculation (EGR) system is used for controlling the generation of undesirable pollutant gases and particulate matter in the operation of internal combustion engines. Such systems have proven particularly useful in internal combustion engines used in motor vehicles such as passenger cars, light duty trucks, and other on-road motor equipment. EGR systems primarily recirculate the exhaust gas by-products into the intake air supply of the internal combustion engine. The exhaust gas which is reintroduced to the engine cylinder reduces the concentration of oxygen therein, which in turn lowers the maximum combustion temperature within the cylinder and slows the chemical reaction of the combustion process, decreasing the formation of nitrous oxides (NoX). Furthermore, the exhaust gases typically contain unburned hydrocarbons which are burned on reintroduction it the engine cylinder, which further reduces the emission of exhaust gas by-products which would be emitted as undesirable pollutants from the internal combustion engine.
When utilizing EGR in turbocharged diesel engine, the exhaust gas to be recirculated is preferably removed upstream of the exhaust gas driven turbine associated with the turbocharger. In many EGR applications, the exhaust gas is diverted directly from the exhaust manifold. Likewise, the recirculated exhaust gas is preferably reintroduced to the intake air stream downstream of the compressor and air-to-air aftercooler (ATAAC). Reintroducing the exhaust gas downstream of the compressor and ATAAC is preferred due to the reliability and maintainability concerns that arise if the exhaust gas passes through the compressor and ATAAC. An example of such an EGR system is disclosed in U.S. Pat. No. 5,802,846 (Bailey), which is assigned to the assignee of the present invention.
A turbocharger as described above typically includes a turbine having a fixed geometry inlet which receives exhaust gas from the exhaust manifold for driving the turbine wheel in the turbine. Since the inlet has a fixed geometry, the rotational speed of the turbine wheel, and in turn the rotational speed of the compressor wheel, is principally dependent upon the flow characteristics of the exhaust gas from the exhaust manifold. The pressure ratio of the compressed combustion air outputted from the compressor thus may not be varied to any significant extent. This in turn may limit the effectiveness of mixing the compressed combustion air with the exhaust gas.
With conventional EGR systems as described above, the charged and cooled combustion air which is transported from the ATAAC is at a relatively high pressure as a result of th charging from the turbocharger. Since the exhaust gas is also typically inducted into the combustion air flow downstream of the ATAAC, conventional EGR systems are configured to allow the lower pressure exhaust gas to mix with the higher pressure combustion air. Such EGR systems may include a venturi section which induces the flow of exhaust gas into the flow of combustion air passing therethrough. An efficient venturi section is designed to xe2x80x9cpumpxe2x80x9d exhaust gas from a lower pressure exhaust manifold to a higher pressure intake manifold. However, because varying EGR rates are required throughout the engine speed and load range, a variable orifice venturi may be preferred. Such a variable orifice venturi is physically difficult and complex to design and manufacture. Accordingly, venturi systems including a fixed orifice venturi and a combustion air bypass circuit are favored. The bypass circuit consists of piping and a butterfly valve in a combustion air flow path. The butterfly valve is controllably actuated using an electronic controller which senses various parameters associated with operation of the engine.
With a venturi section as described above, the maximum flow velocity and minimum pressure of the combustion air flowing through the venturi section occurs within the venturi throat disposed upstream from the expansion section. The butterfly valve is used to control the flow of combustion air to the venturi throat, which in turn affects the flow velocity and vacuum pressure created therein. By varying the vacuum pressure, the amount of exhaust gas which is induced into the venturi throat of the venturi section can be varied. However, inducing the exhaust gas into the flow of combustion air in the venturi throat may affect the diffusion and pressure recovery of the mixture within the expansion section of the venturi.
The present invention is directed to overcoming one or more of the problems as set forth above.
In one aspect of the invention, an internal combustion engine is provided with an exhaust manifold, a turbocharger and a bypass venturi assembly. The turbocharger includes a turbine and a compressor, with the turbine having a variable geometry inlet coupled with the exhaust manifold, and the compressor having an outlet. The bypass venturi assembly includes a housing having an outlet, a combustion air inlet and an exhaust gas inlet. The combustion air inlet is coupled with the compressor outlet. The exhaust gas inlet is coupled with the exhaust manifold. A center piece is positioned within the housing and is in communication with the combustion air inlet. The center piece defines a combustion air bypass section therein. A combustion air bypass valve is positioned in association with the combustion air bypass section. The exhaust gas valve is positioned in association with the exhaust gas inlet.